__proto__ converts to <var></var>proto__

[jmdyck]

ecmarkdown converts, e.g.

  <p>__proto__</p>

to

  <p><var></var>proto__</p>

ecmarkdown should know that a variable can't have an empty name, and thus that an underscore immediately followed by another underscore can't be the start of an UnderscoreFormat.

And similarly with other format-characters*, though none of them occurs doubled in the ES6 spec. Or at least, not in a context that triggers this behavior.

(*Actually, <p>||</p> causes ecmarkdown's parser to throw a TypeError.)

[bterlson]

This is fixed.